1. Field of the Invention
The present invention relates to an electric power steering system.
2. Description of the Related Art
Japanese Patent Application Publication No. 2010-155598 (JP 2010-155598 A) describes an electric power steering system (hereinafter referred to as an “EPS”) that applies torque of a motor to a steering mechanism of a vehicle to assist a driver in performing a steering operation. The EPS performs feedback control for a motor current in order to generate an appropriate assist force that matches at least steering torque. That is, the EPS adjusts the voltage applied to the motor through adjustment of the duty of PWM control in order to reduce the difference between an assist current command value computed on the basis of at least the steering torque and a motor current detection value.
Because higher safety is required for the EPS, the EPS according to JP 2010-155598 A adopts the following configuration. That is, when the steering torque and the assist current command value coincide with each other in direction, the EPS restricts the assist current command value to a determined upper limit value or lower limit value. When the steering torque and the assist current command value are opposite to each other in direction, in contrast, the EPS determines that an abnormality has occurred in assist control computation, and restricts the assist current command value to zero.
However, the EPS according to JP 2010-155598 A involves the following concern. That is, the EPS according to JP 2010-155598 A cannot restrict the assist current command value to zero when the steering torque is in a range of small values (a certain positive and negative range around 0). In general, the assist current command value is generated by superimposing a compensation amount for adjusting the behavior of steering on a fundamental component based on the steering torque. However, the compensation amount may not coincide with the direction of the steering torque. In the case where the steering torque is large, the compensation amount is canceled by the fundamental component even if the compensation amount does not coincide with the steering torque in direction. Therefore, the assist current command value itself coincides with the steering torque in direction. Thus, a disagreement in direction between the assist current command value and the steering torque can be considered as an abnormality in assist control computation.
When the steering torque is in a range of small values, however, the fundamental component is smaller, and the proportion of the compensation amount in the assist current command value is larger. Therefore, the assist current command value and the steering torque may not coincide with each other in direction even if the assist control computation is normal. If the assist current command value is restricted to zero in such a case, the behavior of steering may not be adjusted. Thus, in the EPS according to JP 2010-155598 A, the assist current command value is not restricted to zero when the steering torque is in a range of small values, and the assist current command value is restricted within a range with such a margin that the compensation amount is not restricted. Therefore, even if an abnormal assist current command value is erroneously derived through computation because of some factors, there is less strict restriction on the assist current command value in a region in which the steering torque is small. Therefore, an unintentional assist force is applied to the steering mechanism to occasionally cause self-assist.